Effect of extracellular serum in the stimulation of intracellular killing of streptococci by human monocytes.

This study shows that the intracellular killing of Streptococcus pyogenes, Streptococcus faecalis, and Streptococcus pneumoniae by human monocytes is stimulated by the extracellular presence of both heat-stable and heat-labile serum factors. A similar kind of stimulation of monocytes has been described in respect of catalase-positive microorganisms. However, killing of these bacteria is negligible in the absence of extracellular serum factors, whereas a large proportion of the ingested catalase-negative bacteria are killed in the absence of such extracellular stimuli. Monocytes from patients with chronic granulomatous disease, which are unable to kill Staphylococcus aureus even in the presence of extracellular serum, killed S. pyogenes equally effectively whether serum was present or absent. This index proved to be the same as that for killing by monocytes of healthy subjects in the absence of serum. Taken together, these results indicate that catalase-negative microorganisms possess some kind of suicide mechanism that leads to the death of these bacteria after their ingestion by monocytes in the absence of an extracellular stimulus. Furthermore, the mechanism by which extracellular serum stimulates intracellular killing probably involves enzymes of the O2-dependent bactericidal mechanisms of the monocytes.